The present invention relates to high strength dilute Al-Mg-Si aluminum base alloys and particularly to wrought alloys produced in extruded form wherein strength properties of AA Alloy 6063 are achieved while, at the same time, displaying lower hot flow stresses and reduced quench sensitivity when compared to AA Alloy 6063.
The relative extrudability of an alloy as indicated by permissible extrusion speed, break-out pressure and surface quality is dependent on the hot flow stresses and resistance to tearing and pick-up at extrusion temperature. The extrusion conversion cost is determined, in part, by how fast an alloy can be extruded while maintaining acceptable surface quality. Al-Mg-Si alloys are used to produce 75% of all aluminum extrusions and AA Alloy 6063 has the greatest usage within this class of alloys. It is a common commercial practice to press quench AA Alloy 6063 as the extruded shape leaves the die by employing forced air cooling. In order to obtain the desired strength properties, it is necessary for the alloy to cool through a critical temperature range in less than a certain maximum time period. When the thickness of the wall sections increases, or when extrusion speed is increased, the cooling time under the fans is insufficient to minimize the precipitation of Mg.sub.2 Si. An excessive precipitation of Mg.sub.2 Si during cooling is detrimental because it diminishes subsequent strengthening by a precipitation hardening heat treatment. The cooling time could be reduced by slowing the extrusion rate, by adding additional cooling fans at points farther removed from the extrusion press, or by using a cooling medium like water. All of these solutions are costly or they create other problems such as distortion, water staining, or untenable working conditions. By increasing the allowable time for alloys to cool from the combined extrusion solution temperature and through the critical range (i.e. reducing their quench sensitivity) it is possible to increase extrusion speed and correspondingly decrease the extrusion cost.
Another facet of the problem is that the faster an alloy is pushed through the extrusion die, the higher its temperature rises during extrusion until it reaches a limit where the surface appearance becomes unacceptable, dimensional tolerances cannot be held, or die life is diminished. The rise in temperature is directly related to the hot flow stress of an alloy. In order to raise the rate of extrusion beyond existing limits, it is necessary to minimize the hot flow stress by judicious control of alloying elements or by metallurgical process control.
Accordingly, it is the principal object of the present invention to provide an improved high strength dilute Al-Mg-Si aluminum base alloy characterized by improved extrudability.
A further object of the present invention is to provide an aluminum alloy composition characterized by reduced quench sensitivity at high extrusion speeds.
A still further object of the present invention is to provide an alloy composition characterized by relatively low hot flow stresses.
Another object of the present invention is to provide an aluminum alloy having strength levels comparable to AA Alloy 6063.
Another further object of the present invention is to provide such alloy compositions comprising a dilute range of magnesium in conjunction with an excess of silicon and a copper addition in proportions required to achieve the desired functional characteristics.
Further objects and advantages of the present invention will be apparent from the following detailed description.